2 the Human and Physical Interfaces

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Embedded System and

Applications BNF 30603

1. Introduction to Embedded

System2. The

Human and Physical

Interfaces

3. Serial Interfaces, Timer, Data

Acquisition and Manipulation

4. Interrupts, Delays, and

Multi-tasking

5. Real Time Operating

System

6. ChibiOSReal Time Operating

System

7. Interrupts, Delays, and Semaphores with ChibiOS

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Review: Embedded Systems vs General Purpose Computer

The Human and Physical InterfacesSwitchKeypad LED DisplaysLiquid Crystal DisplaysSome Simple SensorsDigital Input CharacteristicsActuators: Motors and Servos Interfacing to Actuators

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A computer

has..

A microprocessor

Large memory –RAM, ROM, HDD

I/O units

Networking unit

Operating System

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An embedded system also has the same structure but at a smaller size

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A system whose principal function is not computational, but which is controlled by a computer embedded within it [1]

Embedded systems are information processing systems that are embedded into a larger product and are normally not directly visible to the users [5]

Any device that includes a programmable computer but not itself intended to be a general-purpose computer [3]

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Because most devices need control system!

These control systems are embedded within the device, usually unrecognizable

Embedded systems are everywhere

Millions units are produced yearly for various purposes, vs millions of desktop units

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The observation that the number of transistors in a dense integrated circuit doubles approximately every two years. The observation is named after Gordon E. Moore, the co-founder of Intel and Fairchild Semiconductor.

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The Derbot AGV

Derbot AGV Block Diagram 10

Digital Camera Block Diagram 11

HOME OFFICE AND COMMERCE

MOTOR CAR

Washing machine Photocopier Door mechanism

Fridge Checkout machine Brake system

Burglar alarm Printer Engine control

Microwave oven Scanner In-car entertainment

Central heating controller

Climate control

Toys and games

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Source: tech.ubm.com13

GENERAL PURPOSE EMBEDDED

Intended to run a fully general set of applications

Runs a few applications often known at design time

End-user programmable Not end-user programmable

Faster is always better Operates in fixed run-time constraints, additional performance may not be useful/valuable

Differentiating features: • Speed (need not be fully predictable) • Software compatibility • Cost (e.g. RM3k vs RM5k per laptop)

Differentiating features: • Power • Cost (e.g. RM100 vs RM200) • Size • Speed (must be predictable)

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MicroP MicroC

A chip that contains only theprocessor – need other chips tomake a working system

More flexible Can have very few I/O or many I/O

devices using the same processor Used as general-purpose processor

when large embedded software hasto be located in the externalmemory chips

A chip that contains all thecomponents of a computer –processor, memory, I/O

Less flexibility Less component count Less powerful Used when a small or part of the

embedded software has to belocated in the internal memory andwhen on-chip functional units suchas interrupt-handler, timer, etc. arerequired

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COMMON CHARACTERISTICSOF EMBEDDED SYSTEMS

Connected to

physical

environment

through sensors

and actuators

Dedicated user

interface – push

buttons, pedals,

steering wheels

Have to be dependable

Reliability – will not fail

Maintainability – can repair

Availability – reliable + maintainable

Safety – fail, will not cause any harm

Security – secured data

Dedicated towards a

certain application –

extra program

makes the system

less dependable

Real-time constraints

Hard

Soft

Firm

Hybrid systems

– include analog

and digital parts

Have to be

efficientReactive systems

Idle

Input?

Execute

task

Y

N

cost

energy

code-

size

run-

time

weight

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1. Hard RTS

Failure to meet even a single deadline may lead to complete catastrophic system failure

2. Soft RTS

Performance is degraded but not destroyed by failure to meet response-time constraint

3. Firm RTS

Few missed deadlines will not lead to total failure, but missing more than a few may lead to complete or catastrophic system failure

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Example:

1. Air Traffic Control: a service provided by ground-based controllers who direct aircraft on the ground and through controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC worldwide is to prevent collisions, organize and expedite the flow of traffic, and provide information and other support for pilots.

2. Car Engine Management Control

3. Pacemaker: a small device that's placed in the chest or abdomen to help control abnormal heart rhythms. This device uses electrical pulses to prompt the heart to beat at a normal rate.

4. Fire Alarm System

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Example:

1. Automatic Washing machine

2. Air-conditioning

3. CD Player

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Example:

1. Music Sound System in the computer

2. Door Bell Ringing System

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Find two examples of hard, soft and firm real-time embedded system in:

1. Automotive

2. Home Appliances

3. Industrial Automation / Food Industry

4. Medical System

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Unit cost: the monetary cost of manufacturing each copy of the system, excluding NRE cost.

NRE cost (Non-Recurring Engineering cost): The monetary cost of designing the system. Once the systemis designed, any number of units can be manufactured without incurring any additional design cost(hence the term “non-recurring”).

Size: the physical space required by the system, often measured in bytes for software, and gates ortransistors for hardware.

Performance: the execution time or throughput of the system.

Power: the amount of power consumed by the system, which determines the lifetime of a battery, orthe cooling requirements of the IC, since more power means more heat.

Decreasing one may increase another

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Embedded computer reads signals, outputting control signals, interact with human user and possibly interact with an external system via network

Input devices – sensors for measurement or data entry devices for human interaction

Output devices - displays or alarms, and motors or actuators for the physical system

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Extensively used in embedded systems.

Need to convert the switch position to a logic level that can be read by a microcontroller port bit.

Switches are used as direct user interface in the form of push-buttons, toggle switches, slide switches, etc.

Connecting switch to logic input26

Allows numeric or alphanumeric information to be entered.

It is widely used in photocopiers, burglar alarms, central heating controllers.

Arranged in a 4 × 3 matrix, with four rows and three columns

Keypad with pull-up resistors

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Reading a keypad with a microcontroller port (a) Flow diagram (b) Outputs for keypad 28

LEDs made of gallium arsenide (GaAs) emit light in the infrared, and if phosphorus is added in increasing proportions, the light moves to visible red and ultimately to green.

Driving LEDs from logic gates. (a) Gate output sourcing current to LED (b) Gate output sinking current

from LED29

There are eight LEDs in the digit (including the decimal point), but instead of 16 connections being needed, only nine is enough, one for each segment and one for the common connection. The actual pin connections in the example shown lie in two rows, at the top and bottom of the digit. There are 10 pins in all, with the common anode or cathode taking two pins.

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Control is exercised by three control lines:

– Register Select (RS), which determines whether an instruction or character data is being transferred

– Read/Write (R/W), which determines data direction

– Enable (E), which provides a clock function to synchronise data transfer.

The user can access two registers, depending on the state of the RS line:

– An instruction register, used to transfer instructions (RS = 0)

– A data register, used to transfer display data, for example character codes (RS = 1). 31

Large range of sensors available today, which includes ‘smart’ or ‘intelligent’ sensors, which are integrated onto an IC and have on-chip signal processing

Microswitch – mechanical position sensing

Some of Derbot’s sensors and actuators

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Light-dependent resistors (LDR) –made from a piece of exposedsemiconductor material. When lightfalls on it, it creates hole–electronpairs in the material, which improvethe conductivity. When light isremoved, the hole–electron pairsrecombine and conductivity falls. Theoverall effect is that as illuminationincreases, the LDR resistance falls.

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Optical methods are very useful in sensing objects and surfaces. In oneconfiguration the presence of an object can be sensed if it breaks a lightbeam, in another if it reflects the beam.

The reflective optical sensor (a) Principle of operation (b) Electrical connection34

Widely used for sensing and measurement, from simple distancemeasurement to complex medical imaging. The Derbot AGV uses anultrasonic reflective sensor to detect obstacles in its path or to allow it torun parallel to a wall.

The sensor consists of a transmitter and receiver and, to the extent that it isbased on a reflective principle, is initially similar to the reflective opto-sensor.

The big difference lies in the fact that the ultrasound source is pulsed and thetime taken for the echo to return is measured; from this a distance can becalculated.

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If a microcontroller is to receive logic signals, then it is essential that thosesignals are at voltage levels which are recognized by it as being either Logic0 or Logic 1

These voltage levels are usually defined by logic family, for example TTL(Transistor Transistor Logic) or CMOS (Complementary Metal OxideSemiconductor)

When one device is connected to another, and each is supplied by the samevoltage and is of the same logic family, then it is usually safe to assume that logiclevels will be safely and reliably transferred

However, if signals are generated from a non-logic source, e.g. a sensor, or if theyhave been received over a long communication link, or have been subject tointerference, then it may be that they are not correctly interpreted by thereceiver 36

Port bit input voltage levels, 5V supply 37

Different forms of signal corruption. (a) Spikes in signal, potentially harmful to device input.

(b) Spikes in signal. (c) Excessively slow edges. (d) DC offset in signal38

Clamping voltage spikes – with current-limiting resistor Schmitt trigger Analog input filtering Opto-isolation Digital input filtering

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To cause physical movement - linear, i.e. movement in a straight line, or rotary

Solenoids – for linear movement Servos – for angular movement only DC and stepper motor – for angular or rotary Pneumatic and hydraulic – for high forces

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Both can be used for continuous rotary motion or for precise angular displacement

DC motors - range from the extremely powerful to the very small. DC motors drive huge electric trains, but also drive tiny mechatronic systems

Stepper motors - ability to interface very directly with a digital system. Each digital pulse sent to a stepper controller can be used to advance the motor shaft position by a known angle

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Simple DC Switching Only very small electrical loads, like LEDs, can be driven

directly by a microcontroller port bit. Larger loads, drawing beyond 10 or 20 mA, or powered from a voltage higher than the logic supply voltage, need to be interfaced via power switching devices

Transistor switches provide an easy way of switching DC loads – MOSFET and bipolar

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Simple DC Switching

Transistor switching of DC loads.

(a) Resistive, bipolar transistor. (b)

Resistive, MOSFET. (c) Inductive,

MOSFET

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Reversible switching: the H-bridge Some loads, however, for example DC or stepper motors,

need to have a reversible voltage applied, even if only a unipolar supply voltage is available. The way this is usually achieved is by a simple yet ingenious circuit connection called the H-bridge

Two pairs of switching devices, usually transistors, are connected between supply rail and 0V

Each pair has a ‘high-side’ and a ‘low-side’ switch

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Reversible switching: the H-bridge

The principle of the H-bridge

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Serial InterfacesSPI, I2C, USART, USB

Timer

Data Acquisition and Manipulation

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